Hi Tetsujin,

I am not an electrical engineer, but I think you could use a transistor as a switch. Basically feed a higher current source through the transistor to the LED and drive the transistor with the output from the microcontroller. Here is a link that might explain it better:

http://www.rason.org/Projects/transwit/transwit.htm

If you use SMD transistors and resistors, you could keep the size small without the need for 12v power supply provided the supply you do have can provide enough current for all the LEDs you plan to use.

Hope that helps,

Chris